Analysis of Cloud Distribution and its Diurnal Variation over the Tibetan Plateau in Summer Based on Geostationary Satellite Data

The Tibetan Plateau (TP) has a significant impact on the climate at the continental to global scale. During summer, the diurnal variation of clouds over the TP not only affects local convection and precipitation processes but also is closely related to floods in the Yangtze River basin of China, the Asian summer monsoon, and the large-scale climate regime in the Northern Hemisphere. Therefore, understanding the bulk characteristics of clouds over the TP is essential to study cloud-climate feedback and their impact on climate change and dynamics. Due to its remote geographical location, satellite cloud retrievals are highly relied upon for studying the TP. Recent satellite observations have revealed many unique features and roles of clouds over the TP. However, sun-synchronous satellites with low temporal resolution can only capture cloud characteristics twice a day, which is insufficient to understand the diurnal variations of clouds that vary spatially and temporally, especially during summer. The Fengyun-4A (FY-4A) is the first satellite of the new generation of Chinese geostationary meteorological satellites. The Advanced Geosynchronous Radiation Imager (AGRI) on board FY-4A has higher radiation, spectrum, and spatial resolution with shortened revisit time, which provides an opportunity to monitor clouds and their daily cycle with a new level of ability. Although FY-4A/AGRI has the capability to provide a complete image of the TP with smaller viewing zenith angles, cloud retrievals over the TP derived from FY-4A have not yet been exploited and utilized. In this study, pixel-level cloud retrievals obtained from FY-4A/AGRI are used to investigate and analyze the occurrence distribution and diurnal variation patterns of clouds over the TP during summer. The results show that clouds occur most frequently over the south and southeast of the TP all day long, and the TP has an obvious diurnal cycle. Cloud frequencies are highest at noon and lowest at 4 a.m., with peak cloud frequencies distributed along major mountains, showing terrain-dependent characteristics. Cloud tops with a most probable height of more than 12 km are concentrated in the Yarlung Zangbo River Valley and its northern side, near the Nyainqentanglha Mountain Range, the vicinity of Nyainqentangula Mountains, and the west of the eastern boundary of Hengduan Mountains. The diurnal cycle of cloud heights shows an apparent delay against that of cloud occurrence frequencies. This study also discusses the strong topographic influences on the spatiotemporal distribution patterns of clouds over the TP.